Gas turbines are widely used in a variety of commercial operations, such as power generation operations. Gas turbines generally include a compressor, one or more combustors, and a turbine component. Typically, the compressor progressively compresses a working fluid and discharges the compressed working fluid to the combustors. The combustors inject fuel into the flow of compressed working fluid and ignite the mixture to produce combustion gases having a relatively high temperature, pressure, and velocity. The combustion gases exit the combustors and flow to the turbine component where they expand to produce work which may be converted into electrical power.
Liquids forming within the condensed fuel gases may produce serious detrimental effects in the combustors, thereby resulting in hardware damage. Accordingly, conventional fuel suppliers typically provide relatively strict controls to reduce the moisture content of the fuel. However, these conventional fuel supplies typically require additional processing components to ensure that the fuel provided to the combustors is essentially free of liquids.
FIG. 1 shows a simplified diagram of a conventional fuel system 10 for supplying fuel to a gas turbine 12. The fuel system 10 generally includes a supply of fuel 14 having a pressure of approximately 400-700 pounds per square inch. At a given pressure, the fuel may be wet saturated (defined as having a temperature below the hydrocarbon dew point), dry saturated (defined as having a temperature equal to the hydrocarbon dew point), or superheated (defined as having a temperature above the hydrocarbon dew point). The fuel flows through a separator 16, and the separator 16 removes any condensed fluids (e.g., water, condensed hydrocarbons, etc.) from the fuel. A flow control valve 18 throttles the flow of fuel to the combustors of the gas turbine 12. As the fuel expands through the flow control valve 18, the Joule-Thomson effect causes a decrease in the temperature of the fuel. The expansion of the fuel may cause the fuel temperature to fall below the hydrocarbon dew point, allowing condensate to form. To prevent the fuel temperature from falling below the hydrocarbon dew point, the conventional fuel system 10 typically includes one or more heat exchangers 20, 22 upstream of the flow control valve 18. The heat exchangers 20, 22 add heat to the fuel to superheat the fuel and ensure that the fuel temperature remains above the hydrocarbon dew point at all times during the expansion.
FIG. 2 provides a graphical representation of the temperature and pressure changes in the fuel as it moves through the conventional fuel system 10 of FIG. 1. For purposes of illustration, FIG. 2 illustrates the fuel entering the fuel system as superheated fuel, indicated by point A. The heat exchangers 20, 22 heat the fuel to increase the fuel temperature to point B. As the fuel expands through the flow control valve 18, the Joule-Thomson effect reduces the temperature of the fuel from point B to point C. Notably, the gas expansion path from point B to point C remains above the hydrocarbon dew point at all times, preventing condensation in the fuel. The distance between points A and B represents an amount of superheat provided by the heat exchangers 20, 22 to ensure the fuel temperature remains above the hydrocarbon dew point at all times to prevent condensation.
Within conventional fuel system, multiple heat exchangers are typically necessary to ensure that an adequate heat source is available during all levels of operation. For example, during normal operations, the gas turbine 12 may supply the necessary heat. Hot compressed working fluid from the compressor or high temperature exhaust gases from the turbine may be extracted and supplied to one heat exchanger 22 to adequately superheat the fuel. However, during startup operations, heat is not readily available from the gas turbine 12, thus requiring a second heat exchanger 20 with an independent heat source 24.
The need for a second heat exchanger with an independent heat source to supply heat during start up operations requires additional capital costs in the construction of the gas turbine system. In addition, the second heat exchanger typically uses heating coils, an indirect fired heater, a heat pump, or similar devices for providing heat that consumes additional power or fuel during the start up that is typically in scarce supply. Moreover, the power consumed by the second heat exchanger to superheat the fuel decreases the overall efficiency of the gas turbine plant.